The present invention related in general to cyclone furnaces, and in particular to a new and useful cyclone furnace which utilizes tubes having increased outside diameter (OD) and increased wall thickness, for increasing the useful life of the furnace.
A cyclone furnace is a water-cooled horizontal cylinder in which fuel is fired, heat is released at extremely high rates, and combustion is completed. Its water-cooled surfaces are studded, and covered with refractory over most of their area. Coal is introduced into the burner end of the cyclone. About 20 percent of the combustion air, termed primary air, also enters the burner tangentially and imparts a whirling motion to the in-coming coal. Secondary air with a velocity of approximately 300 fps is admitted in the same direction, tangentially at the roof of the main barrel of the cyclone and imparts a further whirling or centrifugal action to the coal particles. A small amount of air (up to about 5%) is admitted at the center of the burner. This is known as "tertiary" air.
The combustible is burned from the fuel at heat release rates of 450,000 to 800,000 Btu/cu ft, hr, and gas temperatures exceeding 3000.degree. F. are developed. These temperatures are sufficiently high to melt the ash into a liquid slag, which forms a layer on the walls of the cyclone. The incoming coal particles (except for a few fines that are burned in suspension) are thrown to the walls by centrifugal force, held in the slag, and scrubbed by the high-velocity tangential secondary air. Thus the air required to burn the coal is quickly supplied, and the products of combustion are rapidly removed.
The release of heat per cu ft in the cyclone furnace is very high. However, there is only a small amount of surface in the cyclone and this surface is partially insulated by the covering slag layer. Heat absorption rates range from 40,000 to 80,000 Btu/sq ft, hr. This combination of high heat release and low heat absorption assures the high temperatures necessary to complete combustion and to provide the desired liquid slag covering of the surface.
The gaseous products of combustion are discharged through a water-cooled re-entrant throat of the cyclone into a gas-cooling boiler furnace. Molten slag in excess of the thin layer retained on the walls continually drains away from the burner end and discharges through a slag tap opening, to the boiler furnace, from which it is tapped into a slag tank, solidified, and disintegrated for disposal.
By this method of combustion the fuel is burned quickly and completely in the small cyclone chamber, and the boiler furnace is used only for cooling the flue tapped into the slag tank under the boiler furnace. Thus, the quality of fly-ash is low and its particle size so fine that corrosion of boiler heating surfaces is not experienced even at high gas velocities.
U.S. Pat. No. 3,081,748 discloses an arrangement using multiple cyclone furnaces to service a single boiler. The furnaces each have multiple panels which are supplied with forced circulation fluid in a specific circuit to maximize heat absorption and plant efficiency.
A known boiler using a similar furnace design includes 134 parallel tubes which make up five water circuits covering the sides of the furnace barrel. Each tube is 1.031" OD and has a minimum wall thickness (MWT) of 0.180".
Many utility companies desire thicker tube construction, however for additional sacrificial tube surface material to extend the life of the tubes. This can only be done however, without any significant increase in pressure drop through the cyclone and while maintaining the same overall dimensions. The new furnace construction must fit within the space provided for the old cyclone furnaces, without any modifications to attaching equipment.